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The number of days per week is a prime number...
(reddthat.com)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. The best ones are thoughts that many people can relate to and they find something funny or interesting in regular stuff.
Actually this incentivizes everyone to have the same periodicity to their recurring tasks. It makes it easier to remember others’ habit schedules.